Obtaining lignonanocellulosis from steam exploded cane baggage and evaluation of its effect on the mechanical properties of Kraft paper

Recently, several studies have sought to optimize the production of lignonanocelluloses (LNC) from agro-industrial residues as these sources substantially reduce their production cost. In Brazil, sugarcane bagasse stands out among other residues because it is the main co-product of ethanol production from sugarcane juice and/or molasses (first-generation ethanol). In the literature, few studies have been dedicated to the exploitation of the steam explosion as a technique for obtaining nanocelluloses (NC) from sugarcane bagasse. Its use for obtaining LNC was not identified in our literature review. Thus, due to its apparent originality, the present work was developed to evaluate the potential of steam explosion, followed by partial delignification techniques, enzymatic hydrolysis and mechanical treatment, for the production of LNC from sugarcane bagasse. For this, two groups of sugarcane bagasse were pretreated at 195 °C for two different residence times, 7.5 min (SE-7.5') and 15 min (SE-15') in a batch steam explosion reactor. All substrates were initially delignified with 0.3 mol L-1 NaOH solution at 80 °C for 60 min. Then, each batch was divided into two parts that were bleached with 0.05 mol L-1 NaOCl at 80 ° C, one for 60 min (SE-7.5'-60' and SE-15'-60' ) and another for 180 min (SE-7,5'-180' and SE-15'-180'). All of these samples were pre-hydrolyzed enzymatically using a commercial endoglucanase (Novozymes), followed by mechanical treatment involving 9 passages through a high-pressure homogenizer. For comparative reasons and for reaction control, the same sugarcane bagasse was pre-treated only by the mechanical process, generating the sample that is referred to as mechanically refined bagasse (MRB). All of these materials were characterized in relation to their chemical composition according to the methodology proposed by NREL. The cane bagasse fibers obtained after all these treatments were characterized for their cationic demand, rheological behavior, morphology (by transmission electron microscopy) and crystallinity index (by X-ray diffraction). The following levels of glucans (mainly cellulose), hemicelluloses and lignin were found in these partially delignified materials: 53.8%, 19.7% and 11.6% for the MRB group; 87.8%, <0.1% and 7.2% for the SE-7.5-60' group; 84.3%, <0.1% and 4.8% for the SE-7.5-180' group; 85.9%, <0.1% and 8.4% for the SE-15-60' group; and 82.2%, <0.1% and 5% for the SE-15-180' group, respectively. According to rheological and microscopic analyses, all groups reached nanometric sizes (mean diameter less than 100 nm and aspect ratio greater than 50) for their classification as nanocellulose. The MRB-B and SE-15' groups had the lowest values of fiber diameter (<20 nm), while those of the SE-7.5' group were greater than 50 nm. The values of aspect ratio and crystallinity index were similar between all groups (~100 and ~73%, respectively), with the exception of the SE-15'-180' group (~78.5 and 77%, respectively) due to the co-presence of shorter nanofibers that were classified by microscopy as nanocrystalline cellulose. The presence of nanofibers and spherical lignin nanoparticles was also found in the SE-7,5'-60' group. Compared to the literature, the steam explosion technique decreased the severity of the mechanical process necessary to reach the nanofiber scale and generated nanomaterials with higher crystallinity indexes. These LNCs were then incorporated as reinforcing agents in handsheets of a commercial Eucalyptus sp. kraft pulp, whose properties were measured in tensile, burst and tear tests. There was an improvement in the tensile index for the MRB-B (22.9% to 3%), SE-7.5'-60' (16.6% to 3%) and SE-15'-60' (12.8% to 1%) groups and in tear index for SE-15'-180' (13.6% to 5%), with no detectable changes in burst index. However, contrary to other studies, all handsheets had similar mass recoveries and no need of compatibilizers for handsheet formation. (author)