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 environ-mental challenge that requires mitigation by improving degradability in environmental compart-ments. In this line, a redox-programmable barrier coating is introduced that combines a high-barrier during usage with a post-use redox-triggered hydrophilization mechanism (Scheme 1). Fully delaminated vermiculite (VMT) nanosheets were intercalated with poly(N-acryloyl thio-morpholine) (PNAT30) to form one-dimensional Bragg-stack nanocomposite barrier coatings on poly(lactic acid) (PLA) substrates, yielding highly ordered, 2 μm thin coatings that drastically suppressed oxygen and water vapor transmission to meet state-of-the-art levels for demanding food packaging at elevated relative humidity. Structural Fe(III)/Fe(II) in VMT was furthermore exploited as an intrinsic redox catalyst as followed 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 up-on successive reoxygenation, are proposed to oxidize hydrophobic thioether side chains in PNAT30 to more hydrophilic sulfoxides, as supported by FTIR, Raman spectroscopy, and solid-state NMR. 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 promote increase post-use environmental accessibility and may facili-tate 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 protec-tion with a built-in trigger for post-use hydrophilization in redox-fluctuating environmental com-partments, such as sewage plants, soils or composting stacks.