Water vapor and air barrier performance of sustainable paper coatings based on PLA and xanthan gum

Abstract

New stable water-based polylactic acid (PLA) emulsions with high water content were successfully prepared using two food-grade surfactants combined with high-shear mechanical mixing and ultrasonic treatment. PLA particle size distribution (PSD) was varied from micrometric to nanometric depending on the preparation conditions, as confirmed by digital microscopies and dynamic light scattering (DLS) analysis. The charged state of the PLA particle surface was evaluated using ζ-potential measurements as a significant factor affecting the stability of PLA emulsions. An unstable PLA emulsion with a large particle size has a low negative charge distribution on the surface (−26 mV) compared to the stable emulsion with a high negative charge (−39 mV). This repulsion force was sufficient to prevent flocculation and subsequent aggregation. Thickened PLA emulsions with different amounts of xanthan gum (XG) display a shear-thinning behavior in the investigated concentrations up to 2 wt% under controlled shear conditions, with apparent viscosity values dependent on XG concentrations. PLA's minimal film formation temperature (MFFT) was determined by applying a linear temperature gradient from 23 °C to 120 °C. The drying above the Tg of PLA (58 °C) resulted in clear and continuous films. Finally, the barrier properties of PLA-coated paper revealed that increasing the thickness of the PLA coating enhanced the barrier properties of the base paper considerably. The results of air and water vapor permeability tests revealed that our PLA coating in weights ranging from 10 to 15 g/m2 is suitable for achieving superior overall barrier properties combined with smooth surfaces, which are extremely important for fabricating coated paper products in the paper industry.