Water vapor barrier properties of wheat gluten/silica hybrid coatings on paperboard for food packaging applications

Motivated by the increasing need for new solutions with less environmental impact, in this work we have investigated the benefits of depositing a wheat gluten (WG) coating on paperboard substrates intended for food packaging applications. To overcome the inherent moisture sensitivity of this protein, WG was combined with a silica network obtained by sol-gel chemistry. WG/silica hybrid coatings were characterized in terms of structural, thermal, morphological, surface, and water vapor barrier properties. Spectrometric analysis demonstrated that the organic and inorganic phases interacted primarily through hydrogen bonding. This was also supported by thermal experiments, which revealed a higher Tg measured for the hybrid materials with the higher silica content (114 +/- 1 degrees C and 128 +/- 2 degrees C, respectively) compared to the pure WG material (Tg = 89 +/- 1 degrees C). Scanning electron microscopy showed that the surfaces of the coatings were very smooth, though the presence of pinholes, cracks, fractures, and voids was detected, especially for the silica-rich formulations. Upon deposition of the coatings, the wettability of the bare paperboard increased, as demonstrated by the lower water contact angle values. In addition, hybrid coatings exhibited a higher wettability over the pristine WG coating, which was due to a more intense spreading phenomenon. The deposition of the coatings led to a 4-fold reduction in water vapor transmission rate (WVTR 90 g m(-2) 24 h(-1) at 23 degrees C and 65% relative humidity) of the specific cellulosic substrate tested in this work (WVTR 350 g m(-2) 24-1).