Carboxylic acid groups and fibre bonding

Abstract

It has been known for a long time that carboxylic acid groups in cellulosic fibres increase swelling in fibres and hence this impacts flexibility, bonded area, and strength. In a few recent publications, attention has been drawn to the possibility that carboxyl groups may serve another function. Those located at the fibre surface have an effect on specific fibre-fibre bond strength. The evidence for this possibility is at present indirect. Using a technique developed for this work, pulps were prepared with uniform distribution of carboxyl groups across the fibre cell wall, and with carboxyl groups located primarily at the fibre surface. Using values of light scattering coefficient, tensile strength, and z-bond strength, it was possible to determine the relative importance of increased bonded area caused by swelling, and increased bond strength caused by localized surface effects. It was found that specific bond strength could be enhanced roughly 50% by surface enrichment, and this could exceed the strength increase attributable to enhanced bonded area. Previous work on the effect of carboxyl groups on strength needs reevaluation to determine the extent of the two effects. The mechanism of strength increase by surface carboxylic acid groups is not understood. It may be an ionic effect, but it is possible that the increased localized surface swelling allows more molecular flexibility and more intimate molecular contact and interdiffusion as found in other polymers. In practice, there are opportunities, especially in bleaching or other pulp treatments, for enhancing fibre surface carboxyl group concentration, and these may have practical value. The field looks ripe for exciting developments.