Produção de nanofibras de celulose a partir dos resíduos de casca de aAcácia-Negra (Acacia mearnsii De Wild.)

Abstract

The present work aimed to reuse the black wattle bark residues in order to use it as raw material for the production of nanocellulose. Thus, it was carried out an evaluation of three chemical pretreatments (alkaline, acetosolv, and organosolv) based on its yield and lignin klason residual on material. The alkaline pretreatment promoted the obtaining of best results, leading to a complete removal of klason lignin. Therefore, it was developed an experimental factorial design 2³ aiming to evaluate the influence of parameters concentration (4-8% m/v), temperature (50- 80°C) and reaction time (1.5-3.5 h) in obtaining of cellulose rich material. This study indicated that the best results were achieved by the employment of NaOH solution 6% (m/v), at 65°C for 2.5 h, allowing a 90.94% reduction in klason lignin content and a yield equal to 63.24%. The bleaching step promoted the obtaining of a rich-cellulose material with a yield of 78.28±1.48$ without the presence of lignin. Thermogravimetric analysis indicated a reduction in the lignin and hemicellulose content, also suggesting an increase in the crystallinity of the material after the employment of each pretreatment step. XRD analysis suggested the presence of type I cellulose, as well as corroborated the removal on non-cellulosic compounds and the increase in crystallinity after the pretreatments. SEM analysis confirmed the alkaline pretreatment efficiency, indicating morphological changes consistent with described earlier. Aiming to use eco-friendly procedures, it was observed that the HIUS procedure promoted the obtaining of cellulose nanofibrils by the ultrasonication of a suspension of microcrystalline cellulose 1% (wt%), at room temperature for 30 minutes. The yield obtained was equal to 3.01±0.52%, promoting the obtaining of nanoparticles with diameter in the range of 9 to 28 nm, as indicated by the atomic force microscopy. It was possible to notice the presence of functional groups that characterizes CNF by the FTIR analysis, as well as the reduction of some bands due to the HIUS mechanism. Regarding the XRD analysis, it was notice that the crystalline planes (011), (-111), (022) and (-231) were maintained indicating the presence of cellulose type I. It was also observed a reduction on crystallinity index of 88.14 to 52.21% which was related to the ultrasonic procedure. Thermogravimetric analysis indicated a reduction on thermal stability related to an alteration on material crystallinity, as well as to the drying method. Finally, it was possible to conclude that the use of black wattle bark residues as raw material for cellulose obtaining is a potential alternative related to its reuse. However, it was detected the need for further studies about the effect of high intensity ultrasound experimental conditions on crystalline and morphological properties of cellulose nanofibers.