Dual Mechanism of Dry Strength Improvement of Cellulose Nanofibril Films by Polyamide-epichlorohydrin Resin Cross-Linking

The chemical and physical properties of cellulose nanofibril (CNF) films cross-linked with wet strength cross-linker polyamide-epichlorohydrin resin (PAE) were investigated. In addition to the expected wet strength improvement, a unique stress-strain behavior for dry cross-linked films was observed. This stress-strain behavior indicates that varying stress-bearing mechanisms arise from the formation of new bonds caused by cross-linking at different levels of cross-linker addition. Cross-linked films showed an approximate 150% increase in dry strength, when compared to non-cross-linked CNF films. The cross-linked films showed a transition of both surface and bulk properties from hydrophilic to hydrophobic. The water contact angle increased from 50 degrees to 110 degrees, while the water retention value decreased by approximately 25% for the cross-linked films. Moreover, water vapor permeability also showed a 2-fold decrease. Possible mechanisms for the changes in behavior are discussed in detail.