High-strength waterborne polyurethane reinforced with waxy maize starch nanocrystals

High-strength elastic nanocomposite materials were successfully prepared from waxy maize starch nanocrystals as the reinforcing phase and waterborne polyurethane (WPU) as the matrix by casting and evaporating. Sulfuric acid hydrolysis of waxy maize starch granules yielded the crystalline platelets of the starch nanocrystals with an average equivalent diameter of 25-40 nm. The morphology, thermal behavior and mechanical properties of the nanocomposite films were investigated by means of transmission electron microscopy, wide-angle X-ray diffraction, scanning electron microscopy, ultraviolet-visible spectroscopy, thermogravimetric analysis, differential scanning calorimetry and tensile testing. The results revealed that the pre-dispersing process made a good effort on dispersing starch nanocrystals into the WPU matrix as small aggregates, and strong interactions occurred between filler and matrix, leading to the efficient strengthening of the composites. The films having 1 wt% starch nanocrystal, rested for one week in 75% RH, exhibited a significant increase from 0.6 to 3.2 MPa for Young's modulus and from 10.4 to 24.1 MPa for tensile strength. Interestingly, they showed high elongation at break, and remained basically in the range from 1148 to 1136%. Further, the WPU based composites possessed good thermal stability. This work provided a new environmentally friendly pathway to prepare WPU based elastomer with high strength.