Dimensional stability of superheated steam dried paper

The dimensional stability of superheated steam dried paper made from groundwood, thermomechanical, chemithermomechanical and bleached kraft pulps were investigated in terms of hygroexpansivity, drying shrinkage and wet expansion. For all the pulps tested these characteristics of dimensional stability were found to be improved by high temperature superheated steam drying.The hygroexpansion coefficient, the dimensional change relative to paper moisture content, was reduced about 15% by superheated steam drying at 320°C, compared to paper dried in low temperature air. Irreversible shrinkage was reduced by up to 30% for sheets dried in superheated steam. However, these effects on hygroexpansivity were caused mainly by the high drying temperature and the resulting high drying rate regardless of the drying fluid. Drying shrinkage and wet expansion as well were reduced by about 10% and 15% respectively for paper dried in superheated steam.In superheated steam drying the elevated web temperature at the start of drying is believed to affect paper properties through thermal softening of the lignin and relaxation of drying and wet-pressing stresses. Increased fiber bonding with softened lignin accounts for both the increased elastic modulus and decreased light scattering coefficient observed. These were obtained only for sheets from mechanical pulps with initial moisture content of about 0.4 kg-water/kg-fiber or more, consistent with the mechanism of thermal softening of plasticized lignin. High sheet temperature in the falling rate period improves hygroscopicity of paper through reducing its equilibrium moisture content.Superheated steam drying improves the dimensional stability of paper from such pulps, including less anisotropy, thereby enhancing paper quality