Mechanical Properties of Information Papers: The Effect of Softwod Kraft Pulp, 1997 Papermakers Conference Proceedings

Reinforcing highly refined pressure groundwood with softwood kraft pulp is observed to increase tensile stiffness and decrease bulk, so that bending stiffness is not affected. The tensile rupture strain is also increased, and thus the tensile index increases slightly more than the tensile stiffness does. A considerable nonlinearity in the tensile index of reinforced groundwood paper is found as a function of the proportion of reinforcement pulp: small amounts of kraft pulp do not increase strength. Including softwood kraft pulp with hardwood kraft pulp hardly affects tensile properties. The effect of softwood kraft reinforcement pulp on the fracture energy of paper is much more pronounced than its effect on tensile strength. Different kinds of softwood kraft pulps separate themselves most clearly with respect to fracture energy, this being a linear function of fiber length. Since the fracture energy (or toughness) together with flaw size distribution determines the greatest load a paper web can carry, softwood reinforcement pulps might be evaluated according to toughness. The fracture energy appears to be a linear function of the proportion of reinforcement pulp, which indicates that reinforcement pulps might be evaluated from pure kraft pulp handsheets.