High-Optical-Performance Composite Films of Deep Eutectic Solvent Pretreated-Cellulose Nanofibrils and Fibrous Clay Minerals

Cellulose nanofibrils (CNFs) produced via deep eutectic solvent (DES) pretreatment were used, for the first time, to prepare composite films/nanopapers with fibrous clays (sepiolite and palygorskite). Highly transparent films containing up to 50% clay were successfully obtained, avoiding energy-intensive homogenization processes and clay chemical modifications, with absolute transparency losses relative to the transparency of the neat CNF film of ∼15% for 50% sepiolite. Higher transparency losses were found for TEMPO-oxidized CNF and cationic CNF composite films prepared for comparison purposes. Moreover, films with wet micronized sepiolite exhibited higher transparency than those with dry micronized sepiolite or with palygorskite. The combination of very thin, highly functionalized, and dispersible nanofibrils of DES CNF with sepiolite fibers enabled the production of films with a very dense and compact structure, contributing to their high optical performance. The straightforward preparation method and the incorporation of clay minerals allow the reduction of the film production costs.