Preparation and properties of thermoset composite films from two-component waterborne polyurethane with low loading level nanofibrillated cellulose

Thermoset nancomposite films were prepared from a two-component waterborne polyurethane (2K-WPU) incorporated with different contents of high aspect ratio nanofibrillated cellulose (NFC). Effects of NFC addition on viscosity and particle size distribution of the 2K-WPU dispersion were investigated by rotational rheometer and laser particle size analyzer, respectively. Viscosities of the 2K-WPU/NFC dispersions increased sharply and particle sizes of the 2K-WPU/NFC dispersions increased slightly with the increase of NFC content. Fractography images from scanning electron microscopy (SEM) showed rough structures appearing in the nancomposites structure, which corresponded to the microphase separation between NFC nano-filler and the 2K-WPU matrix. This microphase separation phenomenon led glass transition temperature (Tg) and break elongation of the 2K-WPU/NFC nanocomposite films decreased with the increase of NFC content. Tensile tests and dynamic mechanical analysis (DMA) indicated all the 2K-WPU/NFC nanocomposite films showed simultaneous enhancements in modulus and tensile strength, compared with those for the neat 2K-WPU (0 wt%). The enhancements can be attributed to strong interactions resulting from the formation of hydrogen bonds and chemical grafting between NFC nano-filler and the 2K-WPU matrix.