Dynamic mechanical properties of paper: effect of density and drying restraints

Abstract

The tensile dynamic mechanical properties of paper sheets with densities between 300 and 1000 kg m⁻³ have been measured at room temperature in the frequency range 0.1 to 10 Hz. Two series of sheets have been investigated; one which had been restrained during drying and the other which had been allowed to shrink freely. In general, paper was found to be a non-linear viscoelastic material. The dynamic modulus decreased and the mechanical loss factor increased as the strain amplitude of the applied sinusoidal deformation was increased. The non-linear character of paper was more pronounced at lower sheet densities. The modulus increased strongly with increasing sheet density and was always higher for sheets that had been restrained during drying. The sheets were anisotropic and the ratio of the modulus in the machine direction (MD) to that in the cross-direction (CD) was of the order of 2 to 3.5 depending on the density and the drying conditions. The mechanical loss factor (extrapolated to zero strain amplitude) decreased with increasing sheet density and was always higher in the CD than in the MD. The freely dried sheets were characterized by a higher value of the mechanical loss factor, tan δ, than sheets that had been restrained during drying. It is suggested that the influence of the sheet density and the drying restraints on the dynamica mechanical properties of paper is associated with interfibre friction and/or bending and shearing of the cellulose fibres.