OPTIMIZING REINFORCEMENT PULPS BY FRACTURE-TOUGHNESS

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 th e sheet when rewetted, such as when sizing, coating, and printing. In- plane fracture toughness was proposed earlier as a criterion to evalua te the reinforcing potential of softwood pulps. At different refining levels, the fracture toughness of a sheet depends strongly on its tens ile strength and extensibility. Fiber properties and processes that en hance these two characteristics also tend to enhance wet-web strength and rewetted sheet strength. This suggests a simple strategy for maxim izing the reinforcing potential of bleached softwood pulps in papermak ing-refine them to a high tensile strength while maintaining acceptabl e drainage characteristics; this generally results in high sheet exten sibility as well. Ways to obtain high sheet tensile strength are discu ssed.