Drying stresses arise in a paper when it is dried restrained due to shrinkage of the fibers when water evaporates. Beating of the pulp influences for example drying stresses due to increased fiber swelling, which in turn influence mechanical properties of the paper. The objective of this study was to analyze how strength additives and debonders or softeners influence drying stresses and strains in laboratory made papers. An additional objective was to analyze how drying stresses and strains influence the mechanical properties of paper in order to understand the mechanisms for changes in mechanical properties of paper by adding dry strength additives and debonders. Both isotropic handsheets and orthotropic papers were made in the laboratory with an Escher-Wyss beaten, unbleached and washed kraft pulp. A softener, a cationic surface-active agent used for debonding of fluff pulp or a wet strength additive, PAE, was added to the pulp before forming the sheets. The sheets were pressed to 40 and 50% dry solids content before they were dried free, under restrain or in a biaxial dryer. The strains in the machine (MD) and cross direction (CD) were controlled independently during drying in a biaxial dryer while corresponding drying stresses are monitored. Drying stresses were lowest for the sheet containing the softener, higher for the control sheet without additives and highest for the sheet with PAE added. Stiffness of the sheets correlated linearly with drying stresses but neither did tensile strength or strain at break. Fiber joint strength has a significant influence on tensile strength and less influence on fracture toughness and compressive strength.
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