Effect of shear and heat milling of starch on the properties of tapioca starch/cellulose nanofiber composites.

Thermoplastic starch was prepared from tapioca starch (TS) and thermo-mechanical modified TS (ModifiedTS) using glycerol as a plasticizer and cellulose nanofiber (CNF) as a reinforcement. The ModifiedTS was produced using shear-and-heat milling machining (SHMM), which effectively reduced starch crystallinity and transformed the granule shape without a chemical reaction. The modified starch with low crystallinity and numerous active hydrogen bonds clearly showed Vh crystal structure after blending with glycerol. The mechanical properties changed significantly upon addition of 10 wt% CNF into plasticized ModifiedTS, which enhanced the yield strength and decreased the elongation at break. A dynamic mechanical analysis revealed a higher glass transition temperature (Tg) of the ModifiedTS films. Tg for the starch film increased from 9.3 °C of the TS film to 29.8 °C of the ModifiedTS film. These results suggest that SHMM provides the ability to widely adjust the mechanical properties of glycerol plasticized starch materials by enhancing interfacial interactions with CNF.