Sustainable barrier coatings for food packaging with a built-in, redox-activated trigger for surface hydrophilization

Abstract

Plastic pollution, particularly from single-use food packaging, has become a significant environmental challenge that requires mitigation by improving environmental degradability. Along this line, a redox-programmable barrier coating is introduced that combines an effective barrier during use with a post-use redox-triggered hydrophilization mechanism. Fully delaminated vermiculite (VMT) nanosheets were intercalated with poly(N-acryloyl thiomorpholine) (PNAT₃₀) to form one-dimensional Bragg-stack nanocomposite barrier coatings on poly(lactic acid) (PLA) substrates, yielding highly ordered, 2 μm thick coatings that drastically suppressed oxygen and water vapour transmission to meet state-of-the-art levels for high-end food packaging at elevated relative humidity. Structural Fe(III)/Fe(II) in VMT was furthermore utilized as an intrinsic redox catalyst as determined by Mößbauer spectroscopy under anoxic/oxic cycling. Exposure to active biomass of the Fe(III)-reducing Geobacter metallireducens reduced structural Fe(III). Reactive oxygen species (ROS), inferred to form via Fenton-type reactions upon successive reoxygenation, are proposed to oxidize hydrophobic thioether side chains in PNAT₃₀ to more hydrophilic sulfoxides, as supported by FTIR, Raman, and solid-state NMR spectroscopy. This molecular transformation triggered a pronounced, stepwise decrease in water contact angle and surface restructuring, evidencing in situ formation of a more hydrophilic coating surface. This is expected to increase post-use environmental accessibility and may facilitate eco-corona development and microbial attachment on derived microplastics (MPs), although direct degradation experiments were not performed in the present study. The concept of coupling mineral-induced tortuous-path barrier enhancement with ROS-activated sulfur chemistry provides a generally novel strategy for designing packaging materials that combine excellent in-use protection with a built-in trigger for post-use hydrophilization in redox-fluctuating environments such as sewage plants, soils or composting stacks.