Wet paper that has been crumpled into a ball shows little tendency to recover to a planar shape when the applied pressure is released – a characteristic called poor wet resiliency. We report the results of an investigation into approaches to improve paper wet resiliency through the choice of fiber types and fiber treatments. Following the lead of the textile industry and the patent literature, wet recovery angle (WRA) was used as a measure of wet resiliency. In this technique, wet strips of paper are folded, without creasing, pressed, and then released. The WRA was measured after the paper relaxed – the greater the WRA, the more resilient the paper.
All of our sheets with a WRA > 0° contained PAE, a standard wet strength resin. Except for sheets based on Abaca fibers, standard PAE or PAE + CMC (carboxymethyl cellulose) applications gave very low WRA. Instead, conventional fibers had to undergo wither TEMPO oxidation or CMC grafting before PAE application. Both fiber treatments substantially increase fiber surface charge density, promoting PAE adsorption and, more importantly, giving covalent bonding sites on the fiber surfaces for grafting PAE.
When comparing the WRA values from treated fibers (TEMPO oxidation or CMC grafting), the ranking was Abaca >>Lyocell > bleached southern softwood kraft > bleached eucalyptus kraft. For pulp mixtures, treated fiber contents of 20-40% had a much bigger influence on WRA compared to wet tensile strength.
Wet-resiliency is due to the swelling of the “hinge region” in folded wet paper, in combination with a sufficiently strong fiber network that can translate the swelling forces into shape recovery. Limited data suggests that the extent of recovery from z-directional wet compression is directly correlated with WRA values. By contrast, wet tensile strength is weakly correlated with WRA.