The influence of drying strategies on the anisotropy of paper in terms of network mechanics

Abstract

The ratio of the tensile modulus in the machine direction of a paper sheet to the tensile modulus in the cross direction (the mechanical anisotropy) is known to be influenced by the fibre orientation distribution as well as by drying restraints. The influence of the drying restraints on the tensile modulus can be described by a network theory recently formulated by Perkins and Mark. According to this model, the deformations encountered during drying increase the modulus of the fibres constituting the network. In the present work, the network theory is applied to a series of paper sheets with different degrees of fibre orientation which are subjected to different drying strategies, such as free shrinkage, restrained shrinkage in both machine (MD) and cross (CD) directions, restrained shrinkage in MD accompanied by free shrinkage in CD and vice versa. The case where the sheet is stretched during drying in MD while free shrinkage is allowed in CD is also included. A satisfactory agreement between the theoretical predictions and the experimental results is obtained in most cases. Some deviations between the theory and the experiments are noted, especially with regard to the shrinkage in CD. This is discussed in the paper.