Does kraft hardwood and softwood pulp viscosity correlate to paper properties?, October 2016 TAPPI JOURNAL

ABSTRACT: The impact of commercially prepared, fully bleached pulp viscosity variation on handsheet physical properties was evaluated at different levels of pulp refining. Hardwood pulps from the same brownstock species mix, cooking parameters, and kappa numbers were processed through two different commercial bleach plants: one with a D₀(EP)D₁D₂ sequence and the second with an OD₀(EOP)D₁ sequence. Additionally, a commercial softwood (predominately Scotts pine) brownstock pulp bleached by an OD₀(EP)D₁D₂ sequence was employed in this study. Pulps with viscosities ranging from 14 to 21 mPa∙s were refined in a Valley beater to two freeness levels, and the associated handsheet physical properties were measured in this study. Over the pulp viscosity range of 14 to 21 mPa∙s, no clear correlation was found to exist between pulp viscosity and related paper physical properties. Finally, a series of laboratory prepared bleached pulps were purposely prepared under non-ideal conditions to reduce their final viscosities to lower values. Handsheets made from these pulps were tested in their unbeaten condition for physical strength properties. Significant and rapid strength loss occurred when the measured pulp viscosity dropped below 12 mPa∙s; overall strength properties showed no correlation to viscosity above the critical 12 mPa∙s value.

Application: For bleached board grades, bond strength appears to play a much larger part in final paper strength than fiber strength, as long as the fiber strength is above a critical value.  Optimization of refining and enhancing fiber bonding should be investigated instead of intrinsic fiber strength when attempting to explain variation in final sheet properties.

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