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

## TL;DR

This paper introduces a new method to create transparent composite films using cellulose nanofibrils and clay, without energy-intensive steps.

## Contribution

The first use of DES-pretreated CNFs with fibrous clays to make high-performance composite films.

## Key findings

- Films with up to 50% sepiolite clay showed ∼15% transparency loss compared to pure CNF films.
- DES CNF films with sepiolite had better optical performance than TEMPO-oxidized or cationic CNF films.
- Wet micronized sepiolite improved transparency more than dry micronized sepiolite or palygorskite.

## Abstract

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.