Rheological response of polylactic acid dispersions in water with xanthan gum

In this work, the rheological behaviour of stable PLA dispersions in water, intended for coating applications, was investigated. The newly prepared dispersions consist of PLA particles with average diameter of 221.9 ± 2.0 nm based on DLS and SEM analyses, at concentrations varying in the 5-22 wt % range. Xanthan gum (XG), a bacterial polysaccharide, was used as a thickening agent to modulate the viscosity of the formulations. The rheological properties of the PLA dispersions with different XG and PLA contents were studied in steady shear, amplitude sweep and frequency sweep experiments. Under steady shear conditions, the viscosity of all the formulations showed a shear-thinning behaviour similar to XG solutions in the whole investigated 1-1000 s-1 range, with values dependent on both PLA particles and XG concentrations. Amplitude and frequency sweep data revealed a weak-gel behaviour except in the case of the most diluted sample, with moduli dependent on both PLA and XG content. A unified scaling parameter was identified in the volume fraction (Φ) of the PLA particles, calculated by considering the dependence of the continuous phase density on the XG concentration. Accordingly, a mastercurve at different volume fractions was built through the Time-Concentration Superposition (TCS) approach. The mastercurve describes the rheological response of the system over a wider frequency window than the experimentally accessible one, and reveals the presence of a superimposed relaxation process in the high-frequency region.