Rheological Response of Polylactic Acid Dispersions in Water with Xanthan Gum

Sara Buoso; Giada Belletti; Daniele Ragno; Valter Castelvetro; Monica Bertoldo

doi:10.1021/acsomega.1c05382

Rheological Response of Polylactic Acid Dispersions in Water with Xanthan Gum

ACS Omega. 2022 Apr 6;7(15):12536-12548. doi: 10.1021/acsomega.1c05382. eCollection 2022 Apr 19.

Authors

Sara Buoso¹, Giada Belletti^{2

1}, Daniele Ragno², Valter Castelvetro³, Monica Bertoldo^{2

1}

Affiliations

¹ Institute of Organic Synthesis and Photoreactivity-Italian National Research Council, via P. Gobetti, 101, Bologna 40129, Italy.
² Department of Chemical, Pharmaceutical and Agricultural Sciences, University of Ferrara, via. L. Borsari, 46, Ferrara 44121, Italy.
³ Department of Chemistry and Industrial Chemistry, University of Pisa, via G. Moruzzi, 2, Pisa 56124, Italy.

Abstract

In this work, the rheological behavior of stable poly(lactic acid) (PLA) dispersions in water, intended for coating applications, was investigated. The newly prepared dispersion consists of PLA particles with an average diameter of 222 ± 2 nm based on dynamic light scattering (DLS) and scanning electron microscopy (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 behavior 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 behavior except in the case of the most diluted sample, with moduli dependent on both PLA and XG contents. 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 master curve at different volume fractions was built using the time-concentration-superposition approach. The master curve describes the rheological response of the system over a wider frequency window than the experimentally accessible one and reveals the presence of a superimposed β relaxation process in the high-frequency region.