Plasma Sputtered Tungsten Oxide Thin Film on Poly(lactic acid) for Food Packaging Applications

Biodegradable and bio-derived plastics such as poly(lactic acid) (PLA) are a promising solution to solve the huge environmental and economic issues caused by the enormous consumption of conventional oil-derived polymers, especially in food packaging applications. However, their poor gas barrier properties and high transparency to UV radiation limit their currently commercialization. Therefore, this study is focused on the deposition of tungsten oxide (WOx) thin films on commercial PLA in order to enhance its overall performance. Coatings with different thickness (25, 50 and 100 nm) were deposited by means of radiofrequency (RF) plasma magnetron reactive sputtering. Morphological characterization was carried out with atomic force microscopy (AFM) and scanning electron microscopy (SEM). In order to evaluate surface chemical changes due to plasma treatments, Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis were performed. The PLA/WOx samples demonstrated remarkable improvements both in UV protection and oxygen barrier properties. In particular, light transmittance was reduced by approximately 95% in the UV-B region, 70% in the UV-A region and 50% in the visible region compared to pristine PLA. Regarding oxygen permeation, a reduction of at least 99.9% was achieved. In addition, the PLA/WOx antibacterial properties against Escherichia coli were also investigated, showing a reduction greater than 5 log10 CFU cm-2 after 24 h for the 50 and 100 nm samples. These results demonstrate the potential of WOx thin coating for sustainable food packaging applications.