The fiber recalcitrance of commercial Eucalyptus pulps affecting the effectiveness of pretreatments and the production of cellulose nanofibrils

Abstract

The principal objective of this study was to evaluate the influence of different Eucalyptus sp. commercial pulp properties on the pretreatment efficiency and its impact on the CNFs production process. To achieve this, two distinct types of pulps were used: bleached kraft pulp, and an unbleached kraft liner pulp with high lignin content. In the first step, the detailed chemical and morphological properties of the materials were characterized. In the second stage, alkaline pretreatments with sodium hydroxide (NaOH) were performed on the bleached pulps at different concentrations and reaction times to obtain fibers with different hemicellulose contents. In the third stage, for the unbleached pulps, two enzymatic pretreatments were used in a sequence: The first with a laccase enzyme aiming to degrade lignin without removing it and the second with an endoglucanase enzyme to hydrolyze cellulose. The CNFs were obtained by mechanical nanofibrillation using a nanofibrillator grinder and the energy consumption was monitored. Subsequently, the obtained CNFs were characterized to understand the influence of the pretreatments on their quality. The present research is divided in two chapters (articles): The first article (Article 1) attempted to evaluate the properties of CNFs with different hemicellulose and cellulose polymorph II contents. A connection was found between these polysaccharides and CNFs properties. A decrease in crystallinity (from 69 to 63%) and changes in the crystalline structure of cellulose subjected to an alkaline environment were observed, promoting partial conversion of cellulose I to cellulose II (from 2 to 42%) and preventing CNFs production at NaOH concentrations higher than 5%. Most treatments showed pseudoplastic fluid behavior, except for the 10% NaOH treatment for 2 hours, which showed Newtonian fluid behavior. The quality index of the 5% NaOH treatment was the highest (68 ± 3 and 22% energy savings compared to the untreated sample), followed by the untreated sample (63 ± 3); and the 10% NaOH treated for 1 and 2 hours had quality indices of 51 ± 3 and 32 ± 1, respectively; The second article (Article 2) aimed to evaluate the impact of a combined enzymatic pretreatment of laccase and endoglucanase to produce lignin-containing cellulose nanofibrils (LCNF). This pretreatment improved the quality of LCNF (61 ± 3 to 71 ± 2) and provided a reduction in energy consumption by 42%. LCNF obtained after the enzymatic pretreatments improved the mechanical strength by 24% and exhibited good water vapor permeability (2.42 g.mm/m². kPa.day) and grease barrier properties (kit no. 12). The rheology of LCNF exhibited non- Newtonian behavior. This study showed that the alkali treatments can promote positive influence on the change in hemicellulose content and cellulose structure treated with 5% NaOH to produce CNFs. Laccase was shown to be effective in attacking lignin, contributing to decreasing the recalcitrance of the cell wall and increasing the exposure of cellulose chains to physical contact for the action of endoglucanase.