Lignin-Containing Cellulose Nanofibrils from TEMPO-Mediated Oxidation of Date Palm Waste: Preparation, Characterization, and Reinforcing Potential

Abstract

Lignin-containing cellulose nanofibrils (LCNFs) have emerged as a new class of nanocelluloses where the presence of residual lignin is expected to impart additional attributes such as hydrophobicity or UV-absorption. In the present work, LCNFs with a lignin content between 7 and 15 wt% were prepared via a TEMPO-mediated oxidation as chemical pretreatment followed by high-pressure homogenization. The impact of the carboxyl content (CC) on the properties of the resulting LCNF gel, in terms of lignin content, colloidal properties, morphology, crystallinity, and thermal stability, were investigated. It was found that lignin content was significantly decreased at increasing CC. In addition, CC had a positive effect on colloidal stability and water contact angle, as well as resulting in smaller fibrils. This lower size, together with the lower lignin content, resulted in a slightly lower thermal stability. The reinforcing potential of the LCNFs when incorporated into a ductile polymer matrix was also explored by preparing nanocomposite films with different LCNF contents that were mechanically tested under linear and non-linear regimes by dynamic mechanical analysis (DMA) and tensile tests. For comparison purposes, the reinforcing effect of the LCNFs with lignin-free CNFs was also reported based on literature data. It was found that lignin hinders the network-forming capacity of LCNFs, as literature data shows a higher reinforcing potential of lignin-free CNFs. Nonetheless, the tensile strength of the acrylic matrix was enhanced by 10-fold at 10 wt% of LCNF content.

Keywords: TEMPO-mediated oxidation; lignocellulosic nanofibers; nanocellulose; property.

Figure 1

(A) Carboxyl, lignin, and holocellulose contents, and yield in recovered fibers for the different TEMPO-oxidized DPW. (B) FT-IR spectra of the original DPWs and the different TEMPO-oxidized samples.

Figure 2

TEM images of negatively stained preparations from the supernatant fractions of LCNF-800 (A,B), and LCNF-1200 (C,D).

Figure 3

(A) Visual appearance of the LCNF suspensions. (B) AFM height images of LCNF-800.

Figure 4

(A) Particle-size distribution of LCNFs at pH 5 and pH 9. (B) ζ-potential vs. pH of LCNFs. (C) Contact angle of water vs time on LCNF films. (D) X-ray diffraction profiles of DPW, LCNF-800, and LCNF-1200 thin films.

Figure 5

(A) Frequency sweeps of G’ and G” for LCNF-1200 at different solid contents (1, 0.5 and 0.25 wt%), and (B) the corresponding complex viscosity.

Figure 6

Thermogravimetric (a) and the corresponding derivative thermogravimetric (DTG) (b) curves of the original DPWs, LCNF-800, and LCNF-1200, under air atmosphere.

Figure 7

(A) Variation of storage modulus E’ and (B) tan δ with temperature for nanocomposite films at different LCNF-800 contents. (C) Plot of E_r vs. LCNF-800 content for the nanocomposite films at 70 °C compared to data from the literature on acrylic containing lignin-free CNFs. (D) E_r at 70 °C for nanocomposite films containing 6 wt% LCNF-500, LCNF-800, and LCNF-1200.

Figure 8

(A) Stress-strain plots of neat matrix and nanocomposites films at different LCNF-800 contents and (B) evolution of both strain at break and ultimate strength as function of the LCNF content.