Effects of starch nanocrystals on structure and properties of waterborne polyurethane-based composites

Starch nanocrystals (StN), which possesses a distinct platelet-like structure similar to exfoliated layered silicate, was incorporated into waterborne polyurethane (WPU) matrix at high loading levels to prepare WPU/StN nanocomposites. In our previous report (Chen et al., 2008a), the StN loading level was restricted to lower than 8 wt.% because self-aggregation of StN resulted in its sedimentation during preparation. However, in this work, good dispersion of the StN nanophase in the nanocomposites was observed even when the StN loading level reached 30 wt.%. Furthermore, the resultant composites exhibited prominent enhancement in both strength and Young's modulus, and maintained an elongation of greater than ca. 300%. A StN loading level of 10 wt.% showed the maximum tensile strength (31.1 MPa) and an enhanced Young's modulus, respectively ca.1.8- and 35.7-fold over those of neat WPU. The active surface and rigidity of StN facilitated formation of an interface for stress transfer and contributed to higher stress-endurance. As the StN loading level increased, self-aggregation of StNs resulted in a decrease in strength; however, the rigidity of StN supported an increase in Young's modulus, which was highest in nanocomposites containing 30 wt.% StNs. High performance waterborne polyurethane-based “green” bionanocomposites were thereby established.