MECHANICAL-PROPERTIES OF INFORMATION PAPERS - THE EFFECT OF ADDING SOFTWOOD KRAFT PULP

Reinforcing highly refined pressure groundwood with softwood kraft pul p increased tensile stiffness while decreasing bulk, so there was no n et effect on bending stiffness. Tensile rupture strain increased, thus indicating that tensile strength index increases slightly more than t ensile stiffness index. There was considerable nonlinearity in the ten sile strength index of reinforced groundwood paper as a function of th e proportion of reinforcement pulp, with small amounts of kraft pulp p roducing no increase in strength. Adding softwood kraft pulp to a hard wood kraft pulp had little effect on stiffness and strength. Adding of softwood kraft reinforcement pulp had a much greater effect on the fr acture energy of paper than on tensile strength. Assuming that the fra cture energy (or toughness) together with flaw size distribution deter mines the greatest load a paper web can carry, it may be feasible to e valuate softwood reinforcement pulps based on the fracture energy. The fracture energy of paper appears to be a linear function of the propo rtion of reinforcement pulp, suggesting that reinforcement pulps might be evaluated from pure kraft pulp handsheets.