Enhancing Cellulose Nanofibril Compatibility with Epoxy Resins through a Water-Based Surface Hydrophobization Strategy

Although the high specific strength and modulus of cellulose nanofibrils (CNFs) make them promising for composite reinforcement, their hydrophilicity significantly reduces the CNF-polymer interfacial compatibility and promotes CNF aggregation in polymer matrices, thus impacting the final composite properties. To alleviate this issue, we hydrophobized CNFs with a tannic acid (TA) primer layer and hexylamine (HA) hydrophobe using a quick, one-pot, and fully water-based strategy. The modified CNFs (CNF-TA-HA) had a water contact angle of 100°, which was stable long term when stored in an aqueous suspension. Using straightforward colorimetric assays, we showed that the CNF-TA and TA-HA reactions both followed a distinctive "two-stage" process in which the CNF surface was modified almost instantly, followed by the slow diffusion of reagents into CNF bundles, which is unique to the highly entangled and polydisperse industrially produced CNFs used here. Adding 1% w/w oven-dried CNF-TA-HA to commercial epoxy improved tensile modulus and tensile strength by 36 and 48%, respectively, compared to 1% w/w unmodified CNFs, and improved tensile modulus by 7% compared to the neat epoxy resin. These enhanced properties were achieved despite CNF aggregation induced by aggressive oven drying and low fiber fraction (where additives would normally just act as defects). Our TA-alkylamine hydrophobization strategy is translatable across a wide range of cellulose nanomaterial sizes and morphologies and offers a green pathway toward tailored compatibility (without degrading the CNF properties) to ultimately improve composite performance.