High-Elongation Starch Films by Hydroxypropylation Reaction with Low Glycerol Content

Hydroxypropylation is habitually used to modify starch by grafting bulky side groups onto its hydroxyls, significantly altering its structural, mechanical, and morphological properties. The hydroxypropylation reaction carried out in this study induced the gelatinization of starch, a phenomenon that is rarely observed in the literature due to the reaction conditions. This distinctive outcome is highly promising, as it enables the production of films with a reduced plasticizer content, offering a significant advantage over conventional methods. In this work, the identified presence of poly-(propylene oxide) (PPO) was investigated to enhance the flexibility of hydroxypropylated starch films, increasing elongation at break from 6% to 34% with only 7.5% glycerol (by weight of starch). PPO also accelerated retrogradation kinetics, attributed to its lubricating effect on starch molecules, and altered the surface energy, resulting in an increased polarity at lower molar ratios. These modifications led to films with reduced brittleness and improved mechanical performance. Scanning electron microscopy revealed a transition from brittle surfaces in native starch films to smoother, more ductile surfaces in modified films. This study demonstrates the effectiveness of PPO as a plasticizer, reducing the need for additional plasticizers and energy-intensive processing. These findings highlight the potential of hydroxypropylated starch films for sustainable packaging applications, advancing the development of eco-friendly materials with enhanced properties.