Mechanical properties of woodfree paper sheets at different surface size starch amounts

This paper examines the effects of surface size starch addition on unsized and uncalendered paper sheets’ mechanical properties. The more specific focus was to study such properties as elastic modulus, elongation at break, and bending stiffness as a function of sheet starch content. The study was based on vacuum-assisted impregnation of starch into the sheet structure and drying the sheets under an impingement hood. Differences in starch amounts were obtained by repeating this sequence, which resulted in a set of specimens with varied starch amounts. Applications with corresponding water amounts were also performed as references in order to remove the effect of water and the wetting-drying cycles from the results. The net effect of starch amounts on the studied sheet properties could then be investigated. Results suggest that the surface sizing starch promotes elastic modulus in cross direction, but not in machine direction. In increasing the elastic modulus – and further the bending stiffness of the sheet – starch may merely promote shrinkage potential, which then leads – when shrinkage is not allowed – to increased drying stress that, in turn, leads to an increase in elastic modulus and further to increased bending stiffness. However, when drying shrinkage is allowed, bending stiffness is increased despite a drop in elastic modulus. This is due to increased thickness, which compensates for the loss in elastic modulus. Therefore, the presence of starch affects bending stiffness in two ways: when drying is restricted, starch promotes increased elastic modulus through increased drying stress; with drying shrinkage allowed, bending stiffness increases as a result of increasing sheet thickness. Since a practical papermaking process involves a large variety of shrinkage behaviors in the cross direction, the mechanism behind bending stiffness can vary in the cross direction based on the shrinkage profile across the entire width of the web.