Composites of nanofibrillated cellulose with clay minerals: A review.

Biopolymers-based composites are, in general, environmentally friendly materials, which can be obtained from renewable sources. Some of them can also present promising properties to be used in food packaging and electronic devices, being thus logical substitutes to petroleum-based polymers, specifically plastics. Cellulose nanofibrils (CNF) obtained by chemical/enzymatic pre-treatments followed by a mechanical treatment appear as a new suitable biomaterial. However, CNF are still quite expensive materials, due to the required chemicals/equipment/energy involved, and additionally, they present some limitations such as high hydrophilicity/high water vapour permeability. The combination of CNF with clay minerals, such as montmorillonite or kaolinite, as widely available geo-resources, represents an excellent way to reduce the amount of CNF used, enabling the production of valuable materials and reducing costs; and, at the same time it is possible to improve the characteristics of the formed materials, such as mechanical, gas barrier and fire retardancy properties, if appropriate conditions of preparation are used. Nevertheless, to obtain hybrid CNF/clay composites with superior properties it is necessary to ensure a good dispersion of the inorganic material in the CNF suspension and a good compatibility among the inorganic and organic components. To fulfil this goal, several strategies can be considered, including physical treatments of the suspensions, CNF and clay surface chemical modifications, and the use of a coupling agent. In this review article, the state-of-the-art on a new emerging generation of composites (films, foams or coatings) based on nanofibrillated cellulose and nanoclay, with focus on strategies for their preparation and most relevant achievements is critically reviewed, bearing in mind their potential application as substitutes for common plastics. A third component has been eventually added to these organic-inorganic hybrids, e.g., chitosan, carboxymethylcellulose, borate or epoxy resin, to enhance specific characteristics of the material. Some general background on the production of different types of CNF and their main properties is previously provided.