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° to 110°, 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.