Optimizing Reinforcement Pulps By Fracture Toughness, 1995 Process & Product Quality Conference Proceedings

R.S. Seth

Pulp and Paper Research Institute of Canada

The major use of bleached softwood kraft pulps is to reinforce weaker papermaking furnishes. Reinforcement is needed not only to enhance the strength of the wet web and the dry sheet, but also the strength of the sheet when rewetted, such as occurs in sizing, coating and printing. In-plane fracture toughness was proposed earlier as a criterion to evaluate the reinforcing potential of softwood pulps. At different refining levels, the fracture toughness of a sheet is shown to depend strongly on its tensile strength and extensibility. Fibre properties and processes that enhance these two characteristics also tend to enhance wet-web strength and rewetted sheet strength. This suggests a simple strategy for maximizing the reinforcing potential of bleached softwood pulps in papermaking - refine them to a high tensile strength while maintaining acceptable drainage characteristics; this generally results in high sheet extensibility as well. Ways to obtain high sheet tensile strength are discussed.