It has long been standard practice to evaluate and assess the usefulness of a papermaking pulp by beating it in a laboratory beater for increasing periods of time and examining the pulp and sheets of paper made from it as the beating process continues. The examination of the pulp can include the inspection under a microscope, fibre fractionation, the Schopper-Riegler evaluation or equivalent, and the water retention value using a standardised centrifuging technique. The examination of the sheets can include any of the physical tests that are of interest for the particular use to which the pulp is to be put. In particular, the rate at which the wetness (in °SR) increases with the beating time is called beatability of the pulp. For example, sulphate pulp is generally more difficult to beat than sulphite pulp, and cotton linters are more difficult still.
This method of pulp evaluation is not only very time consuming, but also does not tell the papermaker much about how best to treat his pulp in the stock preparation. There is a need for a simple method to examine an unbeaten pulp and to assess its behaviour in a beating process without actually carrying out the beating. Such a method can be based on the solubility of the pulp in a suitable solvent. The condition for the success of such a method is that it is sensitive to those structural features of the fibre which affect the beating and to the changes which they suffer during this treatment.
The structural features of the fibre most sensitive to the beating action whilst contributing to the paper properties, are the elements which influence the swelling phenomena in water. Moreover, they are responsible for the solubilisation of low polymers (hemicelluloses) in NaOH-solutions.