Mechanical properties of information papers: the effect of adding softwood kraft pulp

Reinforcing highly refined pressure groundwood with softwood kraft pulp increased tensile stiffness. Tensile rupture strain increased, thus indicating that tensile strength index increases slightly more than tensile stiffness index. There was considerable nonlinearity in the tensile strength index of reinforced groundwood paper as a function of the proportion of reinforcement pulp with small amounts of kraft pulp producing no increase in strength. Adding softwood kraft pulp to a hardwood kraft pulp had little effect on stiffness and strength. Addition of softwood kraft reinforcement pulp had a much greater effect on the fracture energy of paper than on tensile strength. Assuming that the fracture energy (or toughness) together with flaw size distribution determines the greatest load a paper web can carry, it may be feasible to evaluate softwood reinforcement pulps based on the fracture energy. The fracture energy of paper appears to be a linear function of the proportion of reinforcement pulp, suggesting that reinforcement pulps might be evaluated from pure kraft pulp handsheets. Application: softwood kraft fiber reinforcement is effective in papermaking and converting operations where the fracture energy is a crucial material property. The effect of reinforcement on other mechanical properties seems much more moderate.