Structure-property relationships in shellac-coated paper: impact of coating parameters on high-barrier bio-based...

Paper coated with bio-based materials has attracted significant interest as an alternative to plastic-based materials for food packaging, with poor moisture barrier of coated paper posing a challenge to broader applications. Shellac is a promising biopolymer because of its low cost, biodegradability, and wide approval as a food additive and food contact material. In this work, we have systematically studied the role of shellac concentration in butanol and the number of coatings on coated paper's moisture and oil barrier properties. We investigated surface, physicochemical, and thermal properties of coated paper to elucidate specific interactions between shellac and paper substrate and characterize coating morphology, and their effect on barrier properties of coated paper. We found that every layer of coating resulted in an improvement in surface properties, with porosity decreasing by 90 % and surface roughness decreasing by 80 % in a single coat of 15 % (w/w) shellac solution. A low concentration of shellac (7.5 % (w/w)) was found to be less effective in improving barrier properties compared to the highest shellac concentration of 15 % (w/w) used in our study, but still caused a significant increase in the GSM (change in coated weight (g.m^-2)). Four coatings of the 15 % (w/w) shellac solution resulted in a water vapor transmittance rate (WVTR) of 56 g m^-2 day^-1, a water barrier (COBB 180) of 1 g m^-2, and an oil barrier (KIT) of 12. These results meet the strictest requirements for very high-barrier packaging applications.