Transparent and Robust All-Cellulose Nanocomposite Packaging Materials Prepared in a Mixture of Trifluoroacetic Acid and Trifluoroacetic Anhydride

Abstract

All-cellulose composites with a potential application as food packaging films were prepared by dissolving microcrystalline cellulose in a mixture of trifluoroacetic acid and trifluoroacetic anhydride, adding cellulose nanofibers, and evaporating the solvents. First, the effect of the solvents on the morphology, structure, and thermal properties of the nanofibers was evaluated by atomic force microscopy (AFM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA), respectively. An important reduction in the crystallinity was observed. Then, the optical, morphological, mechanical, and water barrier properties of the nanocomposites were determined. In general, the final properties of the composites depended on the nanocellulose content. Thus, although the transparency decreased with the amount of cellulose nanofibers due to increased light scattering, normalized transmittance values were higher than 80% in all the cases. On the other hand, the best mechanical properties were achieved for concentrations of nanofibers between 5 and 9 wt.%. At higher concentrations, the cellulose nanofibers aggregated and/or folded, decreasing the mechanical parameters as confirmed analytically by modeling of the composite Young's modulus. Finally, regarding the water barrier properties, water uptake was not affected by the presence of cellulose nanofibers while water permeability was reduced because of the higher tortuosity induced by the nanocelluloses. In view of such properties, these materials are suggested as food packaging films.

Keywords: All-cellulose nanocomposites; packaging material; robustness; transparency; trifluoroacetic acid; trifluoroacetic anhydride.

Figure 1

(A) Atomic force microscopy (AFM) topographies of short nanofibers (sNF) and long nanofibers (lNF) before and after the solvent treatment. The amorphous domains and agglomerations in the topography of the treated lNF are indicated. Scale bar = 400 nm. (B) Histograms showing the width (top) and length (bottom) distributions of the cellulose nanofibers: sNF (red), lNF (black).

Figure 2

(A) X-ray diffraction (XRD) patterns of sNF and lNF before and after the trifluoroacetic acid/trifluoroacetic acid anhydride (TFA/TFAA) treatment. Main assignments are included. (B,C) Thermogravimetric analysis (TGA) curves and their corresponding derivatives of sNF and lNF, respectively, before and after the TFA/TFAA treatment.

Figure 3

(A) Normalized transmittance as a function of nanocellulose content. Inset: HR-SEM cross-section image of a cellulose sample. (B) HR-SEM cross-section images of lNF30 and sNF30 samples. Scale bar: 500 nm.

Figure 4

(A,B) Stress–strain curves of all-cellulose nanocomposites prepared with short and long nanofibers, respectively. (C) Experimental Young’s modulus as a function of nanocellulose content; the dashed line indicates the analytical model for lNF composites. As the nanocellulose content is increased, the model does not fit the experimental data, even if aggregation is accounted for as a variation of the aspect ratio (hollow points). (D–F) Yield stress, strain at the break, and fracture energy values as a function of nanocellulose content.

Figure 5

(A) Water permeability versus nanocellulose content. (B) Water uptake values versus nanocellulose content.