The Effect of Refining on Yield Stress of Hardwood Pulp, PaperCon 2014

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Yield stress is defined as the shear stress required for flow to take place within a suspension, and it therefore provides an indication of a given suspension’s network strength. In the paper industry, yield stress is an important parameter for both equipment design and paper machine operation, with applications in fiber mat forming, pumping, mixing, and others. Although many studies have been carried out to examine the effects of fiber properties and furnish characteristics on pulp suspension yield stress, little research has been presented on how network strength can be influenced by altering the fiber properties through mechanical treatment, and through refining in particular. In this study, yield stress was determined for Northeastern bleached hardwood Kraft pulp refined with industrial scale equipment under industrially relevant conditions. Specific edge loading (SEL) was used as a parameter to evaluate the effects of refining intensity. Pulp was sampled at various refining specific net energy levels and fully characterized in order to determine the fundamental changes taking place within the fiber suspension and their impacts on overall network strength. High intensity refining was found to decrease pulp suspension yield stress as a result of fiber shortening and reduced fiber stiffness. Alternatively, low intensity refining was found to increase yield stress, which was likely the result of increased interfiber contact strength due to increased fibrillation while maintaining the length and rigidity of fibers.