Comparison of cellulose nanocrystals obtained by sulfuric acid hydrolysis and ammonium persulfate, to be used as coating on flexible food-packaging materials

Cellulose nanocrystals (CNCs), extracted from trees, plants, or similar cellulose-containing materials, can be used in combination with other materials to improve their performance or introduce new applications. The main purpose of this study was to compare and understand the potentialities, as coatings for Poly(ethylene terephthalate) films, of CNCs obtained starting from the same cotton linters by two different processes: sulfuric acid hydrolysis and a less common treatment with ammonium persulfate (APS), able to provide also a cellulose oxidation. The results showed that CNCs produced through the APS treatment showed higher charge densities, due to the carboxylic groups formed during the process, higher crystallinity, higher clarity of the solution and, as a consequence, higher transparency of the coating. These characteristics provide a higher oxygen barrier with respect to the CNCs produced by the H2SO4 treatment, together with the availability of active sites for potential surface modification or chemical grafting. Both CNC coatings showed oxygen permeability coefficients that were lower than synthetic resins commonly used in flexible packaging. Furthermore, they did not significantly affect the optical properties of the substrate, while revealing good friction coefficients. Due though to the moisture sensitivity of the coating and its non-sealable nature, similar to EVOH or PVOH oxygen barrier synthetic resins, CNCs developed using APS will need to be laminated with another plastic layer such as a polyolefin. They could then be used to enhance the final properties of packaging solutions as an alternative to conventional food-packaging materials for perishable food products, while reducing their environmental impact with a thin layer of a bio-based polymer.