Effects of bentonite on physical, mechanical and barrier properties of cellulose nanofibril hybrid films for packaging applications

There is an increasing attention to cellulose nanofibrils (CNFs) for food packaging applications due to their abundance, biodegradability, and low gas permeability. In this work, oxygen and water barrier performance is studied for bio-nanocomposite films formed by incorporation of two types of bentonite (PGN and PGV) at different loads (15, 30 and 45 wt%) into continuous CNF matrix. The resulting hybrid films were analyzed for their morphology, surface energy, mechanical strengths as well as water/oxygen barrier qualities. Both types of bentonite lowered the CNF degradation temperature and strength to some degree for reasons not so clear but perhaps due to partial disruption of the CNF H-bond network. It was revealed from microscopic study that clay particles form a layer within cellulose chains, resulting in alteration of composite structure. The contact angle analysis by polar and nonpolar liquids, suggested the PGN-containing samples were more hydrophilic; clay induced polar functionalities to the composite. While 15% PGN load reduced the water vapor transmission rate from 425 to 375 g/m2 day, higher proportions of bentonite negatively affected this trend. Also, analysis of oxygen transmission rate showed the PGN effectively restricted the oxygen passage in dry state and to a lower extent at higher relative humidity. In WVTR analysis, PGN showed a superior performance over PGV attributable to its crystalline structure as evident in XRD patterns. The proposed hybrid CNF-BNT films in this study can present an eco-friendly alternative in packaging materials, especially where penetration of water vapor and oxygen is to be avoided.