Dried Xanthan Gum/Nanocellulose for Thermoplastic Starch Reinforcement

Abstract

Nanocellulose fibres are known to enhance the mechanical properties of biopolymers by acting as a reinforcing additive. However, the formation of irreversible hydrogen bonds and subsequent agglomeration upon drying means that nanocellulose is commercially only available as an aqueous suspension containing in excess of 75 % water. This not only increases transportation costs, rendering its use financially unsustainable but also limits its applicability in hydrophobic polymers. Therefore, it is crucial to produce dried and redispersible nanocellulose that retains its properties as a polymer filler to fully realise its potential in this field. In this work, the hydrocolloid, xanthan gum, proved to effectively prevent nanocellulose hornification when added to aqueous suspensions at levels as low as 15 % — based on the mass of nanocellulose — prior to drying. This was confirmed by several complementary techniques including rheology, turbidimetry, SEM- and TEM-imaging, as well as colloidal stability analyses. When xanthan gum and nanocellulose were incorporated into thermoplastic starch, a synergistic effect between the two additive materials on the tensile strength of the resultant biopolymer films was evident. The addition of previously dried nanocellulose/xanthan, in a 4:1 ratio, to thermoplastic starch at a starch-cellulose ratio of 20:1, increased the tensile strength from 5.4 MPa to 23.0 MPa. This confirmed that xanthan gum as a nanocellulose capping agent has the potential to enhance its usefulness as a biopolymer additive.