Spraying Transparent Nanoglass Coatings for Food Packaging

When coating suspensions of large aspect ratio clay monolayers upon drying, the individual nanosheets self-assemble into thin nanoglass coatings due to the large overlapping areas between restacked clay nanosheets. By exchanging the films with flat-lying, hydrophobic organocations, swelling at higher relative humidity is suppressed, and the interlayer space collapses to less than 3 Å, a value smaller than the kinetic diameter of oxygen. Consequently, diffusion along the interlayer space is impeded, resulting in very good gas barriers. The guanidinium-exchanged all-clay coating was found to have transmission rates for oxygen as low as 0.32 cm3 μm m–2 day–1 atm–1 and for water vapor as low as 3.74 g m–2 day–1, both values determined at harsh conditions (38 °C and 90% relative humidity). The barrier is rather insensitive to swelling with water vapor, and the performance of the waterborne coatings is competitive, in both barrier performance and transparency with vapor-deposited coatings rendering them an alternative for challenging food packaging applications.