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

Matteo Pedroni,Stefano Morandi,Tiziana Silvetti,Marco Aloisio,Hao Chen,Milena Brasca,Tersilla Virgili,Silvia Maria Pietralunga,Espedito Vassallo,Giuseppe Firpo,Francesco Ghezzi

doi:10.3390/coatings11111281

Abstract

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.

Highlights

The accumulated thermal stress arises both from gas atoms impinging the growing film and the marked difference between the thermal expansion coefficient of WOx and poly(lactic acid) (PLA)
PLA coated with WOx thin films showed improved properties such as barrier to oxygen, UV light protection and antimicrobial activity
The sputtering deposition technique ensured the fabrication of high quality and homogeneous sub-stoichiometric tungsten oxide coatings

Summary

Introduction

Its permeable nature to gas along with its transparency to UV radiation restrict its use for several applications, especially related to food packaging [6,7]. Overcoming these drawbacks is fundamental to estimate and extend the food shelf life. PLA backbone readily undergoes degradation and weakening when exposed to artificial and sun light UV radiation. This results in a color change of the polymer and further UV light transmittance [7]. It becomes primarily important to enhance the barrier and the UV protective properties of PLA

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Conclusion