6 - Refining and Pulp Characterization

Abstract

Pulp refining is a mechanical treatment of pulp fibers to develop their optimum papermaking properties. One action of refining is the “pumping” of water into the cell wall making it much more flexible. A second action of refining is fibrillation, that is, exposure of cellulose fibrils to increase the surface area of fibers, thereby improving fiber-fiber bonding in the final sheet. A third action is delamination of the cell wall, such as between the primary and secondary layers, which increases the fiber flexibility. The optimum paper properties depend on the product being made. Refining of fibers is very important before making paper from them. As it increases the strength of fiber to fiber bonds by increasing the surface area of the fibers and making the fibers more pliable to conform around each other, which increases the bonding surface area and leads to a denser sheet. During refining individual fibers are weakened and made shorter due to cutting action. With very long fibers of a few species this cutting action improves the formation of the sheet on the paper machine, but in most cases, it is an undesired effect of refining. Consequently, refining is generally a tradeoff between improving fiber to fiber bonding and decreasing the strength of individual fibers. Most strength properties of paper increase with pulp refining, since they rely on fiber to fiber bonding.