Kraft pulp viscosity as a predictor of paper strength: Its uses and abuses

Abstract

For bleached kraft pulps, two factors govern paper strength: the individual fiber strength, the bond strength that adheres the individual fibers together in the paper matrix. Inherent fiber strength is related to the length of the carbohydrate polymers, also known as the degree of polymerization (DP). Average DP (DP) is inferred by performing pulp viscosity measurements. Under certain circumstances during kraft pulping, bleaching, the average polymer lengths can be shortened, resulting in lower pulp viscosity, may indicate fiber damage. Fiber damage typically manifests itself as a reduction in tear strength for well-bonded handsheets. This paper will review the literature on how pulp viscosity can predict paper/fiber strength, how it can be used as a diagnostic tool. It can be a means to monitor pulp quality during pulping, bleaching, as well as to alert when such operations approach a critical threshold. However, viscosity losses must be carefully, judiciously analyzed. Like most diagnostic tools, viscosity measurements can be misused, abused, which can lead to incorrect inferences about intrinsic fiber strength. This review will also cover these misuses. The overall goal is to provide the papermaker a better understanding of what pulp viscosity is, how it correlates to potential sheet strength, what its limitations are. It will be illustrated that when pulp viscosity drops below a critical value, it will indicate an appreciable deterioration in the paper’s tear, tensile strength. Application: This paper attempts to reconcile the disparaging inferences made with kraft softwoods, hardwoods with regard to intrinsic pulp viscosity (as measured by TAPPI Standard Test Method T 230), pulp strength, specifically in regards to tensile, tear index.