Gas Barriers from In Situ Polymerized Poly(ethylene glycol) Diacrylate Clay Nanocomposites for Food Packaging

Hydrogel nanocomposites of poly(ethylene glycol) diacrylate (PEGDA) and clay are commonly known for their good biocompatibility and high oxygen permeability while offering sufficient mechanical stability. Dry nanocomposite coatings of this class of polymers would thus not be expected to show appreciable gas barrier performance against oxygen and, even more unlikely for a waterborne system, against water vapor. Here, we applied a wet coat of a homogeneous, nematic mixture of PEGDA of two different molecular weights (575 and 2000 g mol-1) and a large aspect ratio synthetic clay (hectorite, 75 wt %) onto a polyethylene terephthalate (PET) substrate. The wet coat was then photopolymerized quantitatively, as evidenced by infrared spectroscopy, and then dried. Thin coatings of <5 mu m obtained via this in situ polymerization showed excellent barrier performance with the higher molecular weight material outperforming the lower molecular weight material. The nanocomposite with PEGDA of the molecular weight of 2000 g mol-1 had an oxygen transmission rate as low as 0.0042 cm3 m-2 day-1 bar-1 at 23 degrees C and 65% relative humidity (r.h.) and a water vapor transmission rate of 0.035 g m-2 day-1. The coated PET thus outperforms many high-performance polymers such as polyvinylidene chloride used in the packaging of foods or pharmaceuticals. Even more surprising for a waterborne coating, the nanocomposite barriers proved rather insensitive to swelling by water vapor as evidenced by the oxygen transmission rate of 0.662 cm3 mu m m-2 day-1 bar-1 and water vapor transmission rate of 1.805 g mu m m-2 day-1 bar-1 under harsh conditions of 38 degrees C and 90% r.h. Clearly, these nanocomposites provide a halogen-free, waterborne, biocompatible, and degradable alternative to commonly used high-performance packaging materials.