Mixed oxide hybrid coatings for gas barrier applications in polymeric packaging films

The packaging industry's shift towards recyclable mono-materials necessitates high-performance barrier coatings to replace traditional multi-layer structures that do not hinder recycling streams. This study explored the feasibility of mixed oxide (silica-alumina) hybrid coatings, synthesized through an aqueous sol-gel route, as barrier layers on biaxially oriented polypropylene (BOPP) substrates. Alkoxide precursors were reacted in a water-based solution using HCl as the catalyst, and the resulting sols were deposited by rod coating to form optically transparent layers. The incorporation of polyvinyl alcohol (PVA) was critical, producing a homogeneous, crack-free coating that improved the oxygen barrier by a factor of 12. A subsequent two-layer construct with a PVA topcoat was also evaluated. Despite the excellent oxygen barrier, due to the inherent humidity sensitivity of the hydrophilic moieties of PVA, no significant enhancement in water vapor barrier properties was observed. This research demonstrates a method to achieve effective oxygen barriers using an aqueous sol-gel process, thereby reducing reliance on organic solvents and presenting a novel approach for developing hybrid barrier coatings, advancing the design of more recyclable packaging solutions.