Effect of Oxalic Acid Concentration and Different Mechanical Pre-treatments on the Production of Cellulose Micro/nanofibers

The present work analyzes the production and characterization of cellulose micro/nanofibers (CMNFs) obtained from different treatments. A chemical pre-treatment was performed using oxalic acid at 25 wt.% and 50 wt.%. Besides, a rotary homogenizer or a PFI mill refiner were used for mechanical pretreatments. Then, mechanical fibrillation to obtain CMNFs was performed by pressurized homogenization for 5 and 15 passes. The carboxylation of cellulose by the chemical pre-treatment was confirmed by Fourier Transform Infrared Spectroscopy (FTIR) and the increase in the carboxylic acid group content was determined by conductometric titration of CMNFs. The best results of nanofibrillation yield (76.5%), transmittance (72.1%) and surface charges (71.0 µeq/g CMNF) were obtained using the PFI mill refiner, 50 wt.% oxalic acid and 15 passes. The highest aspect ratio (length/diameter) determined by Transmission Electron Microscopy (TEM) was found using the PFI mill refiner and 25 wt.% oxalic acid treatment, and no significant differences were observed by increasing the number of passes. The aspect ratio was related to the sedimentation and the intrinsic viscosity of the CNMF dilute suspension. The aspect ratio values obtained by sedimentation agree with those of TEM. Moreover, a strong relationship between the intrinsic viscosity [η] of the CMNF dispersions and their aspect ratio (p) was found (ρ[η] = 0.014 p2.3, R2 = 0.99). Finally, the tensile strength of films obtained from CMNF suspensions was more influenced by the amount of CMNF fraction than the aspect ratio.