Natural additives to enhance the barrier properties of nanocellulose films

Abstract

Finding alternatives to fossil-based materials is one of the most important challenges of our time. Since cellulose is the most abundant natural polymer on the planet, its use to solve this challenge would be ideal. In the present work, cellulose nanocrystals (CNC) were mixed with different plasticizers to obtain films with improved properties. To this aim, a first study was conducted in order to find the optimal amounts of plasticizer which maximize the compatibility with nanocellulose. The studied additives were Sorbitol (Sor), Glycerol (Gly), Maltitol (Mal), Xylitol (Xyl), Mannitol (Man), Gellan gum (Gg), and Ethylene glycol (Eg). The addition of these plasticizers to the CNC matrix was expected to improve the film flexibility and workability, which is one of the major limitations of this polymer. The selected plasticizers show similar structure to nanocellulose, with free hydroxyl groups which allow compatibility with nanocellulose, resulting in homogeneous films. The films with different amounts of additives were characterized in terms of barrier properties, crystallinity, SFE (surface free energy), and biodegradability. The barrier properties of the films were analyzed in terms of air, oil, water, water vapor and oxygen permeabilities. Results show that all additives decreased air and water permeance, to a greater or lesser extent. CNC films with Mal, Sor, and Xyl had a better oxygen barrier than the control, showing a total oxygen resistance at RH below 60%. About WVTR at moderate (50%) and drastic (90%) humidity, Mal and Gg showed values below the control. Films containing Mal provided the best barrier properties to oxygen and water vapor transmission. The interaction of the films with other compounds (liquids or adhesives) was observed through SFE. In relation to biodegradability, all the additives increased the biodegradability of CNC-based films when subjected to a biodegradability test under controlled composting conditions. These biodegradability results are relevant in terms of the environmental impact of the films, especially if the films are intended to provide a sustainable alternative to traditional food packaging materials