The influence of strain rate and pulp properties on the stress-strain curve and relaxation rate of wet paper

Sufficient tension of the wet web is crucial for paper machine runnability. As profitability pressures have been increasing, the potential to improve runnability is of interest. The runnability of wet paper is especially important for paper machines running at high speeds, making paper at lower grammage, and using large amounts of filler or recovered paper. The typical location of web breaks depends on the paper grade and paper machine. A break typically takes place when the paper is wet and a fast draw is applied to the web. Because information on wet paper is scarce, especially information on the effect of strain rate on the tensile properties, we investigated the effects of strain rate on the tension-strain curve and relaxation of wet paper and how they depend on pulp type, refining, solids content, and fines. Higher strain rate leads to higher tensile strength, tensile stiffness, and relaxation rate. The initial wet strength and stiffness of the studied pulps, bleached softwood kraft pulp and bleached hardwood kraft pulp, were increased by increasing the solids content, fines, wet pressing, and refining. At a constant strain rate and constant strain, the tension at a certain moment depended strongly on the initial tension and thus on the tensile stiffness.