(W1030-02-11) Ethanol Vapour-Induced Morphological Influences on the Water Barrier Performance of PLA Packaging: A Multi-Component Study

Purpose: Poly-lactic acid (PLA) remains the most widely used biopolymer in the food packaging industry due to its inherent transparency and good mechanical properties, allowing for use in films and more rigid thermoform products. In pharmaceutical packaging applications, PLA can come into contact with water/ethanol mixtures or be exposed to humid environments which favor PLA degradation. Especially with formulations or disinfectant procedures. PLA degradation leads to changes in the mechanical and thermal properties, and morphology, reducing the polymers’ ability to act as an effective gas/vapor barrier, potentially leading to product spoilage.

Methods: In this work, a multi-component permeation study to investigate the permeation of water/ethanol vapour mixtures through the PLA films at varying concentrations and temperatures, in order to reproduce real-world conditions. Vapours were introduced to the top of the membrane and the permeate measured underneath. Further studies were undertaken using dynamic vapour sorption – raman techniques to study water and ethanol vapour uptake, and the associated changes in crystallinity. From this data, the partition and diffusion coefficients of the individual components were calculated to determine the contributions of solubility and diffusion towards permeation.

Results: Preliminary results are consistent with those reported by Auras et al.² where the water vapour permeability coefficient (WVPC) decreases through PLA with temperature, which was attributed to the relaxation of the rigid amorphous fraction (RAF) present at the boundaries of crystalline and amorphous domains. When the WVPC is analysed in the presence of 25% ethanol, we observe an overall increase in the WVPC (~40-60%) and a shift in the temperature at which the RAF contributes to a decrease in permeability (from 35 °C to 45 °C). This shift in the temperature effects is also seen in Iniguez-Franco et al. ^<3 where the addition of an ethanol fraction of 25% resulted in a reduction of the glass transition temperature from >50 °C to >43 °C. The reduction of the T_g was attributed to solvent-induced crystallinity over long periods, as seen by in-situ Raman spectra in the DVS data, whereas in this permeation study it is likely to be a swelling effect over the smaller time scales. Investigations into the partition coefficients revealed a decrease (~25%) in the water partition coefficient with increasing ethanol concentrations, suggesting that ethanol decreases the solubility of water in PLA which results in a lower permeability.

Conclusion: This study presents significant data on PLA stability and morphology changes under real-world conditions. Suggesting that at ethanol concentrations above 40%, the barrier properties of the PLA packaging are reduced dramatically, and that temperature influences this solvent induced effect.

References: 1. Sonchaeng, U. et al. Poly(lactic acid) mass transfer properties. Prog. Polym. Sci. 86, 85–121 (2018).
2. Auras, R. A., Harte, B., Selke, S. & Hernandez, R. Mechanical, Physical, and Barrier Properties of Poly(Lactide) Films. J. Plast. Film Sheeting 19, 123–135 (2003).
3. Iñiguez-Franco, F. et al. Concurrent solvent induced crystallization and hydrolytic degradation of PLA by water-ethanol solutions. Polymer 99, 315–323 (2016).


Figure 1: PLA water vapour permeation coefficient against humidity with and without ethanol present


The impact of high ethanol concentrations on polymer mass uptake and the morphology changes via in-situ Raman