High-Performance Waterborne Polyurethane Coating Based on a Blocked Isocyanate with Cellulose Nanocrystals (CNC) as the Polyol

Cellulose nanocrystal (CNC) based composites have been explored as protective organic coatings for metal surfaces, ceramics, and wood. However, the inherent hygroscopic nature of CNCs hinders the technology from being utilized on an industrial scale as water uptake can result in soft, weak coatings. Herein, a moisture-resistant nanocomposite coating was prepared from CNCs in water and a waterborne blocked polyisocyanate (PIC). The cure temperature, which controls isocyanate deblocking and therefore cross-linking of the composite, was investigated across a wide temperature range (25-150 °C), and the subsequent films were characterized in terms of water contact angle, hygroscopic strain, and coating mechanical properties. Water contact angle measurements revealed a remarkable 3-fold reduction in hydrophilicity by cross-linking CNC with PIC. The hygroscopic strain was reduced by 20 orders of magnitude compared to untreated CNCs at 90% relative humidity (RH), which is also evidenced by static moisture sorption studies showing only a 7% moisture uptake. The mechanical and optical properties of the CNC/PIC nanocomposites were investigated to determine the physical performance of the coated material. Finally, the nanocomposite coatings exhibited a reversible humidity-dependent color change. The humidity response of these materials is potentially useful in humidity sensor applications.