Measuring the Bonding Ability Distribution of Fibers in Mechanical Pulps

Currently, the pulp and paper industry mainly uses average values of mechanical pulp properties to characterize fibers, while printing paper grammages keep decreasing, making every fiber more important for strength, surface, and structure properties. Because fibers are inhomogeneous, average values of the whole pulp may not be enough for proper fiber characterization. This paper reports results from the development of a method to measure the distribution of fiber bonding ability in mechanical pulps. Fibers from two commercial TMPs were fractionated into five hydrocyclone streams, using a four-stage hydrocy-clone system. The fiber bonding ability of Bauer McNett fractions R16, P16/R30 and P30/R50 collected from each stream was analyzed. Five different methods of evaluating fiber bonding ability all showed that fibers were separat-ed in the hydrocyclones according to their bonding ability. Long fiber handsheets of the highest bonding fibers had up to 2.5 times higher tensile strength for the P16/R30 fraction than handsheets from the lowest bonding fibers. We also found that both the degree of fibrillation and col-lapse resistance index (CRI) of the fibers obtained from optical measurements are sufficient to predict quite accu-rately the tensile strength of handsheets made from fiber fractions. Further, we propose how to describe the distri-bution in fiber bonding ability for mechanical pulps, by combining some of these five different methods. A method to calculate fracture toughness of long fiber handsheets based on acoustic emission is also illustrated. A more rapid way to characterize fibers in mechanical pulps with respect to their bonding ability distribution needs to be developed in the future. It appears that it is time to move on from characterizing pulp suspensions and handsheet properties using conventional approaches based on average values.